While, as above indicated, the invention has general application and usefulness in various types of wet processing of a myriad of workpiece surfaces, the principal thrust of the preferred embodiment and particular advantageous use of the invention resides in the field of electroplating, and more specifically for such applications as the electroplating of thin planar workpiece surfaces such as silicon semiconductor wafers and the like though the invention will therefore illustratively be described hereinafter as applied to such usage, it is to be understood that it has decided utility, also, for controlling flow or movement of processing fluids at workpiece surfaces more generally, including as further examples, in electroless plating processes, chemical etching, photo resist coating and stripping, spin-on glass and other dielectric coatings, wafer cleaning processes, and the like. While electro-etching processes and the like, similarly to electroplating, also require electric field control, other processes such as cleaning and the like do not involve the use of electric fields.
Turning, therefore, to electroplating applications as illustrative, such electroplating has been widely used for many years as a manufacturing technique for the application of metal films to many different kinds of structures and surfaces. It has been particularly advantageous in semiconductor or solid-state wafer workpiece manufacturing for the application of copper, gold, lead-tin, indium-tin, nickel-iron, and other types of metals or alloys of metals to the wafer workpiece surfaces. An important requirement of the machines used for such a process is that they be capable of depositing metal films with uniform and repeatable characteristics, such as metal thickness, alloy composition, metal purity, and metal profile relative to the underlying workpiece profile For high-volume manufacturing, it is economical to process workpieces using an automated robotic tool, in which a central robot distributes the workpieces to and from separate processing chambers--commonly referred to as a cluster tool that enables the processing of many workpieces per hour and many workpieces per unit of floor-space occupied by the tool. In the exemplary embodiment of such a cluster tool for electroplating in the field of integrated semiconductor circuit manufacturing, the electroplating is used to apply copper films to silicon workpieces for interconnection wiring, to apply lead-tin solder bumps to the workpieces, and also to apply gold to the workpieces. The process chamber designed for such an electroplating cluster tool addresses the various arts of electroplating, fluid mixing, and fluid control. Various features of such a processing chamber can make its integration into an automated wafer handling cluster tool more efficient and useful for manufacturing. It is to these applications as they relate more specifically to a manufacturing cluster tool wet processing chamber, that the present invention is primarily addressed.
At least three factors, however, make it difficult to design equipment that is capable of producing substantially uniform metal films. First, the plating current spreads out when passing from the anode to the cathode, usually resulting in thicker plated deposits near the outer edge of the workpiece. Secondly, the fluid distribution in the electroplating chamber, particularly at the anode and cathode surfaces, may not be uniform. Non-uniform fluid distribution at the cathode, for example, can cause variation of the diffusion boundary layer thickness across the workpiece surface, which, in turn, can lead to non-uniform plated metal thickness and non-uniform alloy composition. Thirdly, the ohmic potential drop from the point on the workpiece at which the electroplating current enters the workpiece may be non-uniform across the workpiece surface, leading to variation in plating current at the workpiece surface and consequently leading to non-uniform metal film deposition.
The prior art reveals several approaches that have been developed to try to minimize one or more of these sources of non-uniformity in the deposited films, particularly for thin and flat workpieces such as wafers and the like. A common arrangement, for example, is described as a fountain plating chamber, or a "fountain plater" as in Schuster et al U.S. Pat. No. 5,000, 827, embodying a fountain or cup plater wherein the water surface to be plated is positioned face down. To control non-uniformity due to edge effects, a method is disclosed wherein the reduction of deposition rate due to fluid effects and the increase in deposition rate due to electric field effects at the workpiece perimeter are balanced against one another to cause substantially uniform plating across the whole workpiece surface. Unfortunately, however, this arrangement is difficult to incorporate in a machine that automatically loads and unloads workpieces in and from the plating chamber. A patent to Stierman et al, U.S. Pat. No. 5,024, 746, as another example, describes a means of operating with a workpiece facing upward toward the cathode such that bubbles will float upwards from the growing plated metal surface to reduce the effect of entrapped air bubbles blocking metal deposition onto the workpiece. This approach, however, requires workpiece attachment means which are difficult to automate for manufacturing. Unlike either of such prior art approaches, the present invention is designed to provide fluid mixing near the growing metal surface which effectively washes entrapped air bubbles from the workpiece surface and carries them out of the plating chamber, as later more fully explained.
As known to those familiar in the art, the technique of fountain plating requires the providing of a distance between the fluid inlet and cathode workpiece which is similar to or greater than the radius or cross-dimension of the cathode workpiece being plated in order to cause acceptably uniform fluid flow at the workpiece surface. Fluid enters at the bottom of the chamber and flows through the anode toward the cathode workpiece surface. The position of fluid passages in the anode, the position of the anode between the fluid inlet and cathode, and the overall size of the fluid chamber are variables that can be changed to influence the uniformity of the electroplated film. The patent to Lytle et al, U.S Pa. No. 5,391, 285, for example, describes a fountain plating cell wherein the anode, the cathode workpiece and the fluid inlet separation distances can be adjusted to cause uniform flow at the cathode workpiece surface. In an article by T. Lee, W. Lytle, B. Hileman, entitled Application of a CFD Tool in Designing a Fountain Plating Cell for Uniform Bump Plating of Semiconductor Wafers, IEEE Trans. On Components, Packaging, and Manufacturing Technology, Part B. Vol. 19, No. 1, February 1996, p. 131, there is a description of how the fluid chamber size, along with the position of the anode in the chamber, can be optimized for producing the best uniformity of plated deposits on a cathodic wafer surface. An inlet to wafer spacing of 70% of the wafer diameter is shown to be. For a 300 mm wafer, as an illustration, this would be a 210 mm fluid chamber height. While in a cluster tool, the compactness of the plating chamber is important for maximizing the economy of the manufacturing process, such prior art fountain plating chambers are decidedly not economical in this regard. As later more fully explained, the present invention, on the other hand, provides a compact electroplating chamber with uniform fluid distribution that has a vertical dimension that is a small fraction of the wafer diameter and is therefore eminently suitable for economical integration into a manufacturing cluster tool.
Another technique for fluid control near the cathode surface known in the prior art, is that of flowing the plating solution through the anode in specific arrangement to cause the fluid to impinge perpendicularly upon said surface. The patent to Croll et al., U.S. Pat. No. 3,317,410 and the patent to Bond et al, U.S. Pat. No. 3,809, 642, for example, describe the use of a flow-through anode to direct the flow of electrolyte orthogonal to the cathode surface, causing fluid mixing at the surface and thereby reducing ionic species concentration variations at the cathode surface. A related method is disclosed in U.S. Pat. No. 5,514,258 to Brinket, wherein a fluid flow collimator plate is disposed between the anode and the cathode and the aperture shape in the plate is designed to ensure laminar flow at the workpiece surface. Because they require an amount of fluid flow proportional to the cathode area, however, these techniques are not practical for substantially large workpieces such as, for example, 200 millimeter and 300 millimeter silicon wafers for which the present invention, with its adequate fluid agitation near the surface of such large workpieces, is particularly suited.
A modification of this directed - flow technique is disclosed by Grandia et al, in U.S. Pat. No. 4,304,641, wherein a rotating flow through a jet plate anode with nozzles of prescribed position and size is used, uniformly to distribute the plating solution on the workpiece-cathode where the metal film is deposited. An apparatus and method for rotating either the workpiece cathode assembly or the anode jet plate assembly is therein described. Tzanavaras and Cohen, in U.S. Pat. No. 5,421, 987, also use such a rotating anode/jet assembly to cause the plating solution to impinge upon the workpiece in a substantially turbulent manner, but uniformly across the workpiece surface. A high-volume flow of plating solution is thus forced through the rotating jet assembly. In these types of systems, solution is pumped into the plating chamber through a rotating seal, requiring a motor, a bearing and a feed-through assembly outside of the plating chamber. In a high-volume manufacturing cluster tool assembly, however, these components would take up valuable space, reducing the economy of the manufacturing tool. In accordance with the present invention, on the other hand, substantial fluid agitation is achieved without a requirement of high-volume fluid flow and without the use of a rotating fluid feed-through or a motor and bearing external to the plating chamber
Still a further prior art approach to trying to solve these problems, is that of using a paddle arm to agitate plating solution near the cathode surface. The patent to Powers and Romankiw, U.S. Pat. No. 3,652,442 is illustrative of the use of a paddle arm moving linearly across the cathode surface during plating. This method has been refined and has, in fact, become the standard manufacturing method in the field of plating magnetic films in the thin film magnetic head industry. A drawback of this method for application to high-volume silicon wafer manufacturing, however, is that it requires a linear drive system that extends a distance larger than the workpiece diameter from the process chamber in a plane substantially parallel to the workpiece surface. Reynolds, in U.S. Pat. No. 5,683, 564, discloses still another method of using rotational paddle-like motion to create fluid agitation in an electroplating cell. A fluid-powered turbine is unitarily formed with a wiper blade that moves near the workpiece surface to prevent hydrogen bubble accumulation on the workpiece surface. In this approach, the wafer is immersed vertically in a cathode chamber in the plating bath--a disposition unsuited, however, for the kind of rapid loading and unloading of wafers required in a high-volume manufacturing system. In contrast, as later explained, the present invention discloses an apparatus for generating rotational motion to agitate the fluid in the process chamber wherein the apparatus does not extend substantially beyond the walls of the plating chamber and wherein the chamber itself is configured such that wafers may be rapidly loaded and unloaded in a fashion particularly suited to incorporation into a manufacturing cluster tool.
Turning, now, to prior art chamber designs, it is known that fluid motion within a circularly symmetric chamber can be very non-isotropic. If the fluid is stirred in one direction, a Coriolis motion is established. A particularly deleterious feature of this type of motion for electroplating chambers or other precision process chambers is the tendency for lighter particles, such as air bubbles, to be drawn toward the axis of rotation, thereby displacing reactants from the surface and causing non-uniform reaction rates on the workpiece surface. This non-uniformity must particularly be avoided in processes such as the before-mentioned wet chemical etching of the workpiece surface or wet chemical stripping of photo-resist from the workpiece surface. Such a Coriolis pattern in the fluid can, however, be avoided by periodically forcing the fluid to rotate for a short time in one direction and then causing the fluid to rotate in the reverse direction for a short time. This kind of reversing rotation operation is embodied in the present invention to provide for precisely and reliably controlling cyclic rotational motion in the fluid inside the process chamber. While it has earlier been known that magnetic couplings can be used to impart motion to a fluid inside a chamber through the use of an energy source outside the chamber, such as magnetic stir bars and magnetically coupled pumps, this invention provides for novel precise controlling of the reciprocating movement of a mechanical stirring component within the chamber from an energy source outside the chamber, and without using a shaft that must pass through the chamber wall which can involve leakage and other problems.
An important feature of a wet process chamber that is designed for electroplating or electro-etching chambers or the like, moreover, is the capability of the chamber to produce an electric field pattern on the workpiece that is either substantially uniform or can be readily tailored to a desired shape. A number of methods and designs have been previously developed to cause the electric field on the workpiece surface to be substantially uniform. These fall into two main categories. A first proposal has been to dispose a conducting element, commonly referred to as a current "thief", in the same plane as the workpiece so that it substantially surrounds the workpiece. A voltage is applied to the element that may be equal to the cathode voltage or controlled to a different voltage as required to influence deposition on the cathode surface. An example is provided in U.S. Pat. No. 5,620, 581 to Ang, using a wafer holder with an integral "thief" ring. A further prior proposal is to position substantially insulating plates between the anode and the cathode to reduce the electric field flow to specified locations on the cathode surface. In Grandia et al, U.S. Pat. No. 4,304, 641, an annular current deflector is used, connected to the anode jet plate to improve the radial uniformity of current that reaches the workpiece surface. A variation is presented in Tzanavaras et al, U.S. Pat. No. 5,421, 987, wherein a collimating screen of substantially annular shape is employed to tailor the current distribution to alleviate edge and corner effects. Such field-shaping shields of this sort, however, either provide benefit only near the edges of the cathode, or they require a relatively large anode-to-cathode spacing to provide benefit across the whole workpiece cathode diameter, such that they are not readily adapted to shape the field continuously across the diameter of the workpiece cathode.
There are also other specific problems, moreover, that are particularly involved in the electroplating of silicon wafers and the like. One such is created by virtue of the fact that the conductive seed on a wafer may not be of equal thickness at all points on the wafer, for example, the seed may have a radial pattern that is related to the symmetry of the system in which the seed layer was deposited. Another factor resides in the fact that in wafers of relatively large diameter having thin seed layers, the ohmic resistance drop from wafer workpiece edge to center can cause non-uniform electric field distribution across the wafer. An economical means of counteracting this non-uniformity is therefore desirable. The present invention, as also later fully described, also addresses these problems by tailoring the current distribution continuously across the radial dimension of the wafer surface, and in a manner that is not necessarily monotonic along a radius of said surface, and further in a manner such that uniform plated film thickness across the cathode wafer workpiece surface may be achieved within a process chamber height that can be made relatively small, say, for example, about one sixth of the diameter of the workpiece cathode itself